The invention relates to a differential amplifier for amplifying an input signal to an output signal, comprising:
a first input terminal and a second input terminal for receiving the input signal,
a first output terminal and a second output terminal for supplying the output signal,
a first transistor and a second transistor, each having a first main electrode, a second main electrode and a control electrode, the control electrode of the first transistor being connected to the first input terminal and the control electrode of the second transistor being connected to the second input terminal;
a third transistor and a fourth transistor, each having a first main electrode, a second main electrode and a control electrode, the control electrode of the third transistor being connected to the first main electrode of the first transistor and the control electrode of the fourth transistor being connected to the first main electrode of the second transistor; the first main electrode of the third transistor and the first main electrode of the fourth transistor being connected to a first node to receive a bias current, and the second main electrode of the third transistor being coupled to the first output terminal and the second main electrode of the fourth transistor being coupled to the second output terminal,
a fifth transistor and a sixth transistor, each having a first main electrode, a second main electrode and a control electrode, the control electrode of the fifth transistor being connected to the first main electrode of the first transistor and the control electrode of the sixth transistor being connected to the first main electrode of the second transistor; the first main electrode of the fifth transistor and the first main electrode of the sixth transistor being connected to a second node to receive a bias current; the second main electrode of the fifth transistor being connected to the first main electrode of the second transistor and the second main electrode of the sixth transistor being connected to the first main electrode of the first transistor.
Such a differential amplifier is known from Netherlands Patent Application no. 8602892, published on Jun. 1, 1988. Differential amplifiers of this type are also referred to as transconductors and are used inter alia in continuous-time balanced integrator filters for various purposes, such as video filters, equalizers etc. This requires differential amplifiers having a large gain-bandwidth product. Preferably, single-stage differential amplifiers are used in order to obtain a large bandwidth in conjunction with a low supply current. FIG. 1 shows the differential amplifier in accordance with the Netherlands Patent Application, which amplifier has a high gain and a large bandwidth owing to the cross-coupled fifth transistor T.sub.5 and sixth transistor T.sub.6. The cross-coupled transistors T.sub.5 and T.sub.6 provide positive feedback to reduce the conversion resistance of the first transistor T.sub.1 and the second transistor T.sub.2, as a result of which a large transconductance is obtained. However, a side-effect of the cross-coupled transistors is that the input impedance of the differential amplifier becomes negative. At low frequencies this effect is negligible but at high frequencies the filter circuit may become unstable as a result of additional phase shifts in the differential amplifier. A suitable compensation is required in order to suppress such instabilities. A conventional compensation method is illustrated in FIG. 2, which shows only a part of the differential amplifier. An RC series network having a positive impedance is arranged across the input terminals to neutralize the negative impedance of the differential amplifier at high frequencies. Although this known method is effective it reduces the high frequency gain and, consequently, the gain-bandwidth product.